Hydrostatic drive systems are used in particular in fields in which heavy vehicles have to be moved with a high level of precision on challenging surfaces. In principle, a driving power of a diesel engine operating predominantly at constant rotational speed is transferred during this process to axle units via an adjustable hydraulic transmission or directly to individual wheels of the axle units. Such drive systems are used in particular in agricultural machinery or in construction machinery. Due to the often unfavorable ground conditions in these fields of application, a key task of the drive system is to drive the wheels with optimal traction and minimal slip.
In such drive systems, only the wheels of a first axle unit, in particular the front axle unit, are often driven. The wheels of another axle unit, for example the rear axle unit, run concurrently in an undriven manner and are often steerable. The axle units or the individual wheels are driven via adjustable or unadjustable or constant hydraulic motors. The hydraulic motors are supplied with pressure medium via at least one hydraulic pump, which is generally driven by the motor of the drive system.
An approach known from the prior art for maximizing the traction of the drive system is, in essence, the use of a four-wheel or all-wheel drive. In addition, the prior art discloses an anti-slip regulation (ASR) for minimizing slip.
DE 10 2005 060 340 A1 discloses a hydrostatic drive system that is operated in permanent four-wheel drive. A method is also disclosed, which minimizes wheel lock under braking (ABS). It is disadvantageous that it is not possible to switch over from four-wheel drive to two-wheel drive and that no anti-slip regulation (ASR) is provided in driving operation.
DE 42 11 323 discloses a hydrostatic drive system and a method for automatically switching over from four-wheel drive to two-wheel drive. The switchover is implemented in this case according to the driving speed. A disadvantage of this is that no anti-slip regulation (ASR) is provided in driving operation.
DE 10 2007 018 449 A1 discloses a permanent hydrostatic four-wheel drive with ASR. A disadvantage of the proposed solution is the lack of switchover between four-wheel drive and two-wheel drive. If slip occurs at a wheel, a control device reduces both a capacity of the respective hydraulic motor and a capacity of the other hydraulic motors. A disadvantage of this is that the overall driving torque is reduced and that there may be a threat of standstill of the vehicle and hindered breakaway after the standstill.
EP 1582 389 B1 discloses a hydrostatic drive system of a working vehicle with ASR, with which it is possible to switch over between all-wheel drive and two-wheel drive. A disadvantage in this case is that switchover only takes place as required if the driving conditions are monitored continuously by an experienced operator, which is correspondingly costly. As a result, it is likely that the operator will engage four-wheel drive permanently, which is less effective compared to two-wheel drive. In the case of ASR, a control device initially reduces a capacity of a hydraulic motor driving the wheel concerned or the axle unit concerned before accordingly increasing the capacity of the hydraulic motors assigned to the wheels not experiencing slip so as to equalize the volume flow rate balance and to achieve the overall driving torque. A disadvantage of this is that the first response causes the overall driving torque to be reduced initially, which may be associated with a standstill of the vehicle and hindered breakaway after the standstill.
DE 43 96 170 B4 discloses a four-wheel drive for a dump truck comprising a mechanically driven axle unit and a hydraulically driven axle unit. The drive is provided initially via the mechanically driven axle unit. In the event of slip at the wheels of the mechanical axle unit, the hydraulically driven axle unit is activated. A disadvantage of this solution is that two different drive concepts have to be operated matched to one another, which constitutes increased technical complexity of the apparatus.
A disadvantage of the suggested drive systems and of the suggested methods is that an overall driving torque of the drive system is initially reduced when slip occurs.
By contrast, the object of the disclosure is to create a hydrostatic drive system having improved traction when slip occurs and improved efficacy.
This object is achieved by a hydrostatic drive system having the features described below and by a method for the anti-slip regulation of a hydrostatic drive system having the features described below.
Advantageous developments of the disclosure are disclosed in the below description.